Method for processing rough timber

ABSTRACT

The invention relates to a method for timber processing during which the wood, such as sawn wood that is prepared for seasoning is dried in a kiln in a manner known per se at a temperature of between about 40° C. and about 100° C., to a moisture content of not more than 30%. 
     Nitric acid is mixed with an aqueous solution an alkali earth metal salt, and this mixture is sprayed into the drying kiln at least once. The spray loosens the pectin of the fiber that blocks the evaporation of the moisture, while the alkali earth metal is being built into the wood fibers.

FIELD OF THE INVENTION

The present invention relates to a method for processing rough timberwherein the wood, such as sawn wood prepared for seasoning is dried in akiln, at a temperature of between 40° C. and 100° C., to a moisturecontent of not more than 30%.

BACKGROUND OF THE INVENTION

The primary products of timber industry, such as various kinds of sawntimber, veneer, and wood waste, that are produced for further use,usually have a moisture content above the saturation limit of the fiber.In this state timber is not suitable for further processing or shaping,therefore the moisture content has to be reduced by natural orartificial drying to under the saturation limit of the fiber to theair-dry (12-18%) or the room-dry (6-12%) state. The rapid increase inindustrial use of wood products and the need for producing components ofroom-dry state made it essential to accelerate the slow natural dryingprocess that usually lasts for at least 2 to 5 years, by the use ofvarious artificial drying methods.

The acceleration of the natural drying processes (temperature, wind,humidity) has not led in an unchanged form to a good result, thereforevarious methods have been developed to remove the moisture from the woodas quickly and carefully as possible. Nearly all these methods are basedon modifications of the properties and parameters (temperature, speed ofair current, pressure, humidity) of the surrounding air, except for highfrequency drying in which drift of the moisture begins from the inside.

Known artificial drying methods include:

convection drying,

condensation drying,

vacuum drying,

high frequency drying, and

drying in hot oil.

The first three procedures are generally used in practice. Each of themis suitable for reaching the required degree of humidity by changingvarious parameters of time and space. Quick drying of "dry" timber,which is quick compared with natural drying, usually involves three mainphases:

heating,

drying, and

compensation (and conditioning, cooling).

The moisture content of timber dried by the aforementioned methods canbe adjusted to the required level, but

macroscopic features and defects in the wood result in inaccuracy in theachieved value (usually in the order of 1-5%),

wood undergoes different deformations during drying (e.g. bending,warping, splitting, of butt or pith, etc.) because of the acceleratedreduction of moisture content,

tensions of different types and scales emerge during drying and remainuntil processing when further bending and warping take place, and

the effects of the changes in the surrounding humidity, the achieving ofbalance in the moisture content of the wood, and other changes in thehumidity (e.g. moistening) have the result that the artificially driedmaterial changes its shape and dimension to a larger extent thannaturally dried material.

DESCRIPTION OF THE INVENTION

The object of the present invention is to eliminate the aforementioneddeficiencies and to provide a drying method for timber with reduceddeformation, and changes in shape and dimensions, and holding theremaining stresses and the duration of the drying to an acceptableminimum.

During the development of the method of the invention we relied on theknown methods and combined them with chemical processes that have hadsignificant effects but have not been used before. Therefore any dryingtechnique (convection, condensation, or vacuum drying) can be usedsubject to the following considerations.

Accordingly the present invention is a timber processing method whichcomprises drying substantially unseasoned timber while spraying it atleast once with a mixture of a mineral acid and an aqueous solution ofan alkali earth metal salt, whereby the pectin of the wood fiber whichnormally blocks the evaporation moisture from therebetween will becomeloosened to permit evaporation of the water content of the wood, andsaid alkali earth metal is incorporated in the wood fiber.

By the process of the present invention the stiffened state of thepectin in the molecular structure of the timber material is loosened atthe critical moment. This enables the internal moisture to reach thesurface and to evaporate without problems and then the acidic alkaliearth solution promotes the incorporation of the alkali earth metal intothe fibers of the natural material.

Suitably nitric acid is mixed with a solution in distilled water of analkali earth metal salt, and this mixture is sprayed at least once intothe drying kiln. The spray loosens the pectin of the fiber thatotherwise blocks the evaporation of the moisture, while the alkali earthmetal is built into the fiber of the wood.

According to an embodiment of the invention, the mixture to be sprayedcontains 42-48 mass % nitric acid, 42-48 mass % distilled water, a saltof an alkali earth metal, such as 4-16 mass % calcium chloride. Theconcentration of the nitric acid can suitably be from about 65% to about96%. Depending on the nature and the condition of the wood, suitablyfrom about 10 sparing cycles each of from about 2 to about 15 minuteseach can be employed.

Suitably the principal parameters of the present method are

    ______________________________________                                        kiln temperature                                                                             from about 40° C. to about 100° C.               speed of air current                                                                         from about 0.1 to about 10 meters/sec                          pressure       from about 0.5 bar to about 1.063 bar                          duration heating                                                                             from about 3 to 18 hours                                       drying         from about 12 to about 71 hours                                conditioning   from about 0 to about 12 hours                                 moisture content of wood                                                                     from about 50% at the beginning, and                                          between about 6 and                                                           about 15% according to                                                        specification at the end of the process                        ______________________________________                                    

Generally the moisture content is between about 30% and about 8% duringthe progress of the process. The wood suitably containing of from about18% to about 30% before the start of spraying in accordance with thepresent invention.

The dry wood obtained from the process within a few days has the samequalities as wood naturally dried for a number of years. Another resultis that the resulting wood of low moisture content can be shaped, suchas twisted, compressed, elongated, attenuated, defibered, etc, flexibly,without scobs. Therefore it can be formed almost like fresh wood.

The drawbacks of the prior art processes can be minimized or eliminatedby use of the present invention:

(a) the process of the invention produces an effect like artificialaging reducing the changes in the shape and dimensions that arisebecause of the macroscopic features of wood, to a minimum in the courseof drying;

(b) the changes in shape that take place during the known drying methodsare avoided, therefore, the wood that is waiting for processing is notdamaged by bending, warping, or splits;

(c) the dried wood is free of tension, therefore there is no loss ofwood after processing, because all of the wood can be used;

(d) wood can be put to new uses, such as scobless shaping of wood thathad been assumed could not be bent before, or flexible shaping withoutdamages caused by temperature;

(e) the acceleration of growth and wood use in accordance with thepresent invention will avoid problems such as employing wood with amoisture content above the permissible limit, and reusing changes inshape, dimension, etc., thus further damages.

We claim:
 1. A method for drying timber, which comprises dryingsubstantially unseasoned timber in a kiln while spraying into the kilnat least once a mixture of nitric acid and an effective amount of anaqueous solution of an alkali earth metal salt, whereby the pectin ofthe wood fiber which normally blocks the evaporation of moisture fromtherebetween will become loosened to permit evaporation of the watercontent of the wood, and said alkali earth metal is incorporated in thewood fiber.
 2. The timber processing method of claim 1, wherein saidaqueous solution is a solution in distilled water.
 3. The timberprocessing method of claim 2, wherein the timber is dried at from about40° C. to about 100° C. to a minimum moisture content of 30% by mass. 4.The timber processing method of claim 3, wherein said mixture containsfrom about 42 to about 48% by mass (65%) nitric acid, from about 42 toabout 48% by mass water, and from about 4 to about 16% by mass of saidalkali earth metal salt.
 5. The timber processing method of claim 4,wherein said alkali earth metal salt is calcium chloride.
 6. The timberprocessing method of claim 2, wherein the concentration of the nitricacid is from about 65 to about 96% by mass.
 7. The timber processingmethod of claim 2, wherein the spraying is commenced when the moisturecontent of the timber is between about 18 and about 30% by mass.
 8. Thetimber processing method of claim 2, wherein the timber is dried afterspraying, at a temperature between about 40° C. and about 100° C.
 9. Thetimber processing method of claim 2, wherein spraying is carried out ofa period of from about 2 to about 15 minutes depending on the nature ofthe wood of the timber.
 10. The timber processing method of claim 9,wherein said spraying is carried out in from about 1 to about 10spraying cycles.